It is possible to overfill an engine with oil. It can cause some damage, but mainly it will decrease power and fuel economy because the crankshaft is turning through the oil which adds resistance to the engine.

Overfill plug on an A/T? Never heard of one. You add fluid at the same place you check it. Do not overfill.

What problems can occur from engine oil overfill

When you overfill oil in a lawnmower it will smoke when you tip it sideways while mowing,it has no harm

Yes